1. Field of the Invention
The present invention relates to a curl correcting device that corrects curl on a paper sheet, and an image forming apparatus and a sheet post-processing device that employ the curl correcting device.
2. Description of the Related Art
A post-processing device is sometimes connected to an image forming apparatus. The post-processing device aligns each set of image-formed paper sheets that are discharged from the image forming apparatus and performs various post-processes on the paper sheets. Such post-processes include stapling or punching.
The post-processing can not be performed in a desirable manner if the paper sheets are curled. Heat and pressure are applied to paper sheets in the image fixing process performed in image forming apparatuses. The paper sheets get curled when heat and pressure are applied to them.
The paper sheets can curl in a random direction (hereinafter, “curling direction”) depending on the type of material of the paper sheets. However, conventional curl correcting devices can correct curls made on only standard paper sheets. That is, the conventional curl correcting devices cannot adequately correct curls made on, for example, thick sheets or non-standard sheets. On top of this, the conventional curl correcting devices may even curl the paper sheet in an opposite direction if the effect of the correction is too large.
Use of curl corrected/removed paper sheets decreases the number of jams of paper sheets under conveying and allows the preferable post-processing that is a subsequent process of the curl correcting process, which improves the quality of the post-processing significantly.
Various devices that remove curl from paper sheets have been proposed. For example, Japanese Patent Application Laid-open No. 2007-119193 discloses a curl removing device that can correct curls in various directions. Japanese Patent Application Laid-open No. 2000-247526 discloses a curl correcting device that can adjust linear speed depending on the conveyance path and includes a loop space that allows a paper sheet to bend. Japanese Patent Application Laid-open No. H9-6073 discloses a technology in which the amount of curl correction is adjusted depending on the degree of curl.
The conventional technologies require complicated structures for correcting the curl.
For example, the technology disclosed in Japanese Patent Application Laid-open No. 2000-247526 that removes curl depending on the curling direction needs conveyance paths corresponding to different curling directions and a curl correcting roller arranged in each of the conveyance paths in such a manner that the conveyance paths can be switched by operation of a switching claw. Therefore, a large space is needed for arranging the two conveyance paths and the switching claw in the device. There is a demand for a simplified configuration.